The arrangement according to the invention has been primarily developed for fitting and dismantling propeller assemblies of the kind generally referred to as thrusters. This type of propeller assembly normally comprises, in principle, a propeller mounted on a propeller shaft journalled in a gear housing which accommodates a bevel gear mechanism through which the propeller shaft is coupled to a drive shaft which extends through a tubular support strut, one end of which is connected to the gear housing to support the same. The other end of the strut is intended for installation in an opening located in a bottom part of the shell structure or the hull of the watercraft, to enable the drive shaft to be connected to propeller-drive machinery located within the hull. To facilitate mounting of the propeller assembly around the opening in the bottom of the watercraft, the strut has located at the upper end thereof, or in the vicinity of said upper end, a mounting flange which can be bolted firmly to a mounting ring encircling the opening in said shell structure. The strut is often journalled in the mounting flange in a manner which permits the whole of the assembly comprised of the strut, gear housing and propeller to be swung about an axis which coincides with the axis of the drive shaft, and the strut can be coupled to machinery arranged within the hull and adapted to carry out this rotary motion. This enables the propeller force generated by the propeller assembly to be set to any desired direction. Such a propeller assembly is usually called a rotatable thruster.
Propeller assemblies of the aforedescribed kind are being increasingly used for moving and maneuvering, e.g. holding position, of different types of platforms used in the offshore industry. Such propeller assemblies, however, are also used in various types of special-duty watercraft, such as craft equipped for sea-diving purposes, crane-bearing watercraft, cable-laying vessels, and can also be used for moving and maneuvering, for example, floating docks, pontoons and the like. Accordingly, the term "watercraft" used in the aforegoing and in the following text is meant to include all water-buoyant constructions and devices which float in water and which can be moved therein and with which a propeller assembly of the aforementioned kind can be used.
It must be possible to fit and dismantle such propeller assemblies, for servicing, repair and exchange purposes, without needing to take the watercraft concerning into dock. Consequently, it must be possible to fit and to dismantle the propeller assembly with the mounting opening in the bottom of the watercraft beneath the surface of the water. Devices have been proposed and designed with which fitting and dismantling of the propeller assembly can be effected from within the confines of the watercraft, but because of the large dimensions of the propeller assembly and its weight, these devices are highly space consuming, expensive and impracticable. Consequently, it is endeavored to fit and dismantle such propeller assemblies externally of the watercraft in question, i.e. it shall be possible to disconnect the assembly from the drive and rotary machinery from within the watercraft, and to remove the assembly from its mounting around the aforesaid opening and then lift the assembly on the outside of the hull of the vessel, up to the surface of the water. Similarly, it shall also be possible to move the assembly from the water surface externally of the shell structure of the watercraft down to the intended mounting opening in the bottom of the watercraft and there secured to the shell and connected to the drive and rotary machinery within said craft. It is known to use for this purpose and plurality of lines connected to powerful lifting devices located on the watercraft or optionally on an auxiliary craft, and passed through tubes which extend through the interior of the watercraft and out through the shell structure thereof, by the side of the propeller assembly mounting opening. The ends of these lines are connected to the part of the propeller assembly located in the water beneath the shell structure, and subsequent to disconnecting the assembly from the drive and rotary machinery within the watercraft and from the shell structure, the assembly can be lowered by means of the lines and the aforesaid lifting devices, down out of the mounting opening through a distance sufficient to enable the assembly to be lifted clear of the hull of the watercraft to the surface of the water on one side of said hull, with the aid of one or more further lines connected to lifting devices on the watercraft or on an auxiliary vessel. The propeller assembly is fitted to the mounting opening in the reverse order, by first lowering the propeller assembly with the aid of one or more lines externally of the hull of the craft to a position at which the lines extending through the tubes located on the side of the mounting opening can be secured to the propeller assembly and used to lift the assembly and position the same in the mounting opening in the shell of the watercraft. These known arrangements for fitting and dismantling the propeller assembly externally of the craft beneath water level require the use of divers, however, for connecting and disconnecting the lines to and from the assembly. Offshore platforms, however, are often situated in sea areas in which prevailing or expected weather conditions over a large part of the year render diving unsafe. This presents a serious problem, since the periods in which weather conditions are extremely bad or threaten to be so can be extremely prolonged.